Modem society continues to create exponentially increasing demands for digital information, and the communication of such information creates increasing needs for ever-faster data communication speeds. The most common form of computer-to-computer communication in use today relies on modems and analog telephone network connections. The telephone network, however, was designed to provide approximately 3.3 kHz of analog voice bandwidth. Such a bandwidth provides adequate voice communication, at low cost, but does not support high-speed data communications. Integrated Services Digital Network (ISDN) offers somewhat faster data communications and the capacity for concurrent data and voice telephone services. However, this technology has some drawbacks, and data rates offered by ISDN already may be too slow. The high-speed and wide availability of modern personal computers (PCs) continually gives rise to ever more sophisticated multimedia applications. Communications for such applications, typically between the PC and the Internet, are driving the need for speed to rates far above those available on analog telephone lines and on normal ISDN lines.
A number of technologies are being developed and are in early stages of deployment, for providing substantially higher rates of data communication, for example ranging form 640 kb/s to 9 Mb/s. Of particular note, after considering several other options, a number of the local telephone carriers are working on enhancements to their existing copper-wire loop networks, based on various xDSL technologies. xDSL here is used as a generic term for a group of higher-rate digital subscriber line communication schemes capable of utilizing twisted pair wiring from an office or other terminal node of a telephone network to the subscriber premises. Examples under various stages of development include ADSL (Asymmetrical Digital Subscriber Line), HDSL (High data rate Digital Subscriber Line) and VDSL (Very high data rate Digital Subscriber Line).
Consider ADSL as a representative example. For an ADSL related service, the user's telephone network carrier installs one ADSL modem unit at the network end of the user's existing twisted-pair copper telephone wiring. Typically, this modem is installed in the serving central office or in the remote terminal of a digital loop carrier system. The user obtains a compatible ADSL modem and connects that modem to the customer premises end of the telephone wiring. The user's computer connects to the modem. The central office modem is sometimes referred to as an ADSL Terminal Unit--Central Office or `ATU-C`. The customer premises modem is sometimes referred to as an ADSL Terminal Unit--Remote or `ATU-R`.
For digital data communication purposes, the ATU-C and ATU-R modem units create at least two logical channels in the frequency spectrum above that used for the normal telephone traffic. One of these channels is a low speed upstream only channel; the other is a high-speed downstream only channel. Two techniques are under development for dividing the usable bandwidth of the telephone line to provide bidirectional transmission. Currently, the most common approach is to divide the usable bandwidth of a twisted wire pair telephone line by frequency, that is to say by frequency division duplexing. The frequency division approach uses one frequency band for upstream data and another frequency band for downstream data The downstream path is then divided by time division multiplexing signals into one or more high-speed channels and one or more low speed channels. The upstream path also may be time-division multiplexed into corresponding low speed channels. The other approach uses Echo Cancellation. With Echo Cancellation, the upstream band and the downstream band substantially over-lap. The modems separate the upstream and downstream signals by means of local echo cancellors, in a manner similar to that used in V.32 and V.34 modems.
The telephone carriers originally proposed use of ADSL and similar high-speed technologies to implement digital video services, for example in networks sometimes referred to as video `dialtone` networks. The ADSL line technology provided a mechanism for high-speed transport of MPEG encoded video information to video terminal devices in the customers' homes. Examples of such ADSL-based video dialtone networks are disclosed in U.S. Pat. Nos. 5,247,347, 5,410,343 and 5,621,728. Interest in such video services has waned, but the recent explosion in Internet and other PC-based services has sharply rekindled the carriers' interest in xDSL technologies. The carriers are now proposing a range of xDSL data services targeted at high-speed Internet access and high-speed access to private data networks. U.S. Pat. No. 5,790,548 to Sistanizadeh et al. discloses an example of an ADSL based data network, e.g. for high-speed access to the Internet and to corporate LANs.
In the last year or so, considerable attention has focused on one version of ADSL with somewhat reduced capabilities but which does not require a separate splitter/combiner at the customer premises to segregate the telephone traffic from the data traffic. The ADSL `Lite` modem can plug directly into the customer's telephone wiring, without a special installation by a telephone company technician. The customer's `Lite` modem does not need or include a frequency splitter/combiner to segregate the voice and data traffic. The `Lite` modem uses a more restricted frequency band, in order to reduce interference with telephone service. Although this reduces the downstream data rate somewhat, particularly for longer lines, the `Lite` implementation still provides downstream speeds ranging from 640 b/s to 1.5 Mb/s, which are substantially higher than provided by analog modems or ISDN.
Thus, ADSL modems today are providing downstream data rates in ranges from 640 kb/s to as high as 9.1 Mb/s. The precise data rate depends on many factors, such as line length, copper wire gauge. cross-coupled interference, and the like. As a general rule, the shorter the distance, and/or the smaller the wire gauge, the higher the rate can be on the particular telephone line. These rates provide an order of magnitude improvement over telephone line modems and ISDN equipment currently used for Internet or other data network access services.
Installation, operation and maintenance of ADSL-based data services, however, pose a number of problems. These problems may be particularly acute where a carrier is considering upgrading service to ADSL on an existing subscriber's line circuit. As noted, the length and gauge of the wiring can effect performance. If the wiring has been in place and used for telephone service, there may be load coils on the line, which disrupt xDSL services. Bridged-taps, which are common in telephone loop plant, also cause performance problems.
In the telephone industry, twisted wire pair circuits from a central office or a subscriber line carrier unit generally are bridged-tapped along their length, to provide a line appearance in a number of different terminals located at different points along the multi-pair feeder cable. An installer can connect a subscriber's drop line to binding posts in the closest terminal, but the line appearance remains in other terminals connected to the multi-pair cable. At a later date, an installer can disconnect the first subscriber drop line from the one terminal, and connect a new subscriber's drop line from another terminal, in order to reuse the twisted wire pair connection through the feeder cable back to the central office for another subscriber. The presence of bridged-tapping, particularly extended wiring downstream of a particular subscriber's connection to twisted wire pair in a terminal, may cause considerable disruptive interference effects. For example, the extending wiring adds capacitance and resistance. The extended wiring picks up considerable electromagnetic interference from external sources and may pick up cross talk from adjacent active pairs. All of these effects disrupt xDSL broadband digital service on the twisted wire pair.
To install and operate an xDSL modem on a line, the telephone carrier needs to know if the line is in such a state as to enable high-speed downstream transmission at the full rates desired for the particular xDSL service. If not capable at the highest rate, it is usefull to determine what lower rate the line may support. The carrier company can take a number of different steps, if it knows the line capabilities. For example, the carrier may rearrange its line connections to provide a subscriber desiring a particular xDSL service with an adequate line pair.
At present, however, there is no adequate technique for testing loop plant wiring, particularly existing telephone wiring, for its compatibility with the high-speed data services. Often, the carrier must install the modems for the desired service, operate the modems and pray they work. If there are problems, there is no easy way to measure or diagnose specific problems. The carrier's technicians can try a number of different fixes, on a hit-or-miss basis, and retest the modem operations. Such approaches to testing are time consuming and often ineffective.
Many carriers today utilize a mechanized loop test (MLT) system, for analyzing reported troubles on subscribers' telephone lines. An MLT system selectively connects to the central office terminals of twisted pair telephone wiring and conducts electrical tests on metallic circuits. Such a system can apply an AC voltage across a wire pair, between the Tip (T) wire and ground, and between the Ring (R) wire and ground and take appropriate measurements to determine characteristic impedances. The MLT system can also measure the DC resistance between the wires and between each wire and ground. The MLT system stores a list of DC and AC resistance/impedance values that correspond to certain line conditions, e.g. shorts, opens, normal telephone set connections, etc. The MLT system makes decisions as to presence or absence of different types of faults by comparing the test result values to its stored list of values. These MLT tests provide limited information regarding the transfer characteristic of the loop, particularly with respect to the frequency ranges effecting xDSL services.
A need therefore exists for an efficient technique for testing a line to certify the line for a particular high-speed data service, like ADSL, before putting the line into service. A need also exists for a technique to enable testing and maintenance of in-service lines, to enable the carrier to respond to troubles and outages reported by subscribers.